A typical molten metal facility includes a furnace with a pump for moving molten metal. During the processing of molten metals, such as aluminum, the molten metal is normally continuously circulated through the furnace by a centrifugal circulation pump to equalize the temperature of the molten bath. These pumps contain a rotating impeller that draws in and accelerates the molten metal creating a laminar-type flow within the furnace.
A well-known problem with such processes, however, is the accumulation of dross within the metal bath. Dross is a mass of solid impurities floating on and within a molten metal bath. It usually appears within molten metals or alloys having a relatively low melting point, such as tin, lead, zinc or aluminum, or by oxidation of these metals. Other impurities, such as pieces of the furnace's refractory material may also be found within such a molten metal bath.
The dross can range in size from small particles to relatively large pieces or chunks. The smaller dross material, while undesirable, does not normally pose a threat to the operation of the furnace and its circulation pump(s). However, the larger pieces of dross can, and often do, get pulled into the pump and cause the pump to become jammed, causing catastrophic failure to occur.
As a result of this problem, furnace operators frequently must run their circulation pumps at a relatively low speed, such as approximately 250-300 rpm. This slower speed, while reducing the damage to the pump components if a larger piece of dross becomes lodged therein, results in the undesirable condition of the pump motor being operated at much less than peak efficiency. That is, through the use of a frequency converter, a motor can produce the necessary torque at these lower speeds, but resulting in only using 10-15% of the available motor horsepower.
Another common solution to the pump-damaging large dross pieces within a furnace is to install larger and larger pumps having impellers that will receive and transfer all but the largest contaminants. These pumps, however, are expensive and they waste energy by continually pushing the dross through the system. Furthermore, as the dross is circulated through the furnace along with the molten metal, the dross pieces tend to accumulate together or conglomerate to create larger and larger pieces. Eventually, these growing pieces of dross may reach a size that will jam within even a very large impeller.
Currently, there are filters that are placed at the furnace discharge. These filters prevent dross from exiting the furnace—not from entering the pumps. Additionally, they do not provide for a user to collect the filtered contaminants from the system. Instead, the contaminants are left free to settle throughout the furnace.
In view of the current inefficient use of molten metal pumps, there is a need for a system for filtering the larger dross pieces in the furnace from entering a molten metal pump, thereby allowing the pump to operate at higher speeds and increasing efficiency within the system. Additionally, there is a need for such a filtering system that enables a user to quickly clean out the accumulated filtered contaminants from the furnace.